A significant tonnage of metallic scrap typically enters landfills or is re-directed to other manufacturing sectors because it is unsuitable for reprocessing. To be suitable for cost-effective reprocessing, metallic scrap must meet certain physical criteria relating to size, shape and density. Most common higher quality steel and cast iron scrap includes automotive, plate and structural, shredded steel, foundry grade steel, busheling steel and cold and hot pressed cast iron borings.
Select examples of scrap traditionally not seen as suitable for reprocessing include steel shot fines and dust. Other scrap grades such as cast iron borings/chips and steel turnings have value, but not in an unfinished form. These types of materials often are first pre-processed into briquettes, and then are further reprocessed.
Recycling is often accomplished by melting the scrap material in a cupola, induction furnace, arc or blast furnace. The choice of melting process is partially dictated by the physical properties of the charged scrap in conjunction with the final desired metal chemistry at tap.
Most recently the foundry industry has experienced extreme volatility in terms of availability and pricing for cupola raw materials. As such it has become important to attempt to utilize other types of previously little-utilized raw materials such as steel shot fines/dust or loose borings. These often are not easily reprocessed due to problems with the melting process caused by their shape or small size.
The sizing distribution for these little-utilized raw material grades are commonly ½″ or less, and in some instances measuring finer than minus 70 mesh (−70M).
When scrap is this excessively small a variety of detrimental process conditions are created such as:
(a) handling issues,
(b) metal yields,
(c) reduced melt rates, and
(d) recovery rates.
Handling
Handling issues will arise since it is not practical to transport raw materials ranging in size from ½″ by down in the loose shape or form (i.e., as -produced condition). One can expect a reduction in metallic yield due to losses in transport, both upon delivery and on-site movements, and quite possibly due to inclement weather during storage such as high winds.
To offset these issues it will be necessary to pre-package the material in possibly super sacs, bins, totes or boxes. This will only augment costs and the need for more storage space and frequency of material movement. This implies that these raw materials in their as-produced condition are too unmanageable for efficient charging and melting in a cupola.
Yield Losses
Further metallic losses are expected upon cupola charging. It is observed that as loose fines/chips descend down the cupola stack the exiting top gases will re-direct some of the scrap material to the air pollution control system. Not only does this have a direct impact on metallic yield but it simultaneously promotes higher maintenance and disposal costs.
Reduced Melt Rates
As fines/chip-sized scrap descends down the cupola stack they tend to fill in the voids between the larger-sized scrap promoting a bridging effect. This restricts the upward gas flow and consequently increases the wind-box back pressure. If this occurs it may be necessary to reduce the blast volume to alleviate the condition—which in turn reduces metal output.
Recovery Rates
Sizing and surface area are interrelated. It is an inverse relationship. As the sizing decreases, the surface area increases. This characteristic normally promotes higher oxidation losses which causes a reduction in metallic recovery rates.
The past technology trend to alleviate the above identified pitfalls associated with small-sized scrap (i.e., in loose shape or form) has been to reshape or repackage the scrap, primarily by a process termed briquetting. The prior art teaches two main methods for briquetting—hot/cold pressed briquettes, and bonded briquettes.
Hot/Cold Briquettes
Metallurgically speaking there are essentially three (3) grades or families of cast iron; namely, gray, malleable and ductile. The borings from these respective grades are normally hot or cold pressed to form a briquette shape. Borings are simply machining fragments from the castings. The fragments themselves appear as metallic chips either in the wet or dry state dependent on the machine shop's tooling practice.
One ideally prefers to process dry borings since no cost is attached for the removal of the water and oil coolants. The mechanical/physical properties of the two briquetting techniques are quite polarized. The cold pressed briquettes yield an inferior product since the briquettes are much weaker and tend to deteriorate causing both high handling and melting losses. In the hot pressed briquetting process, the borings are elevated from ambient temperature to levels as high as 1500°F. It is quite possible at this elevated temperature to oxidize the carbon, silicon and iron elements if a proper reducing atmosphere is not maintained. At this temperature the borings are softened to promote a dense and strong briquette bond that will not deteriorate during handling. Overall, the hot pressed briquette is more desirable.
Bonded Briquettes
Certain raw materials such as steel shot fines/dust are not conducive to hot or cold-pressed process due to their physical properties. In order to re-shape or re-package this type of material a “bonding agent” is required. Bonding agents are normally cement or possibly some chemical compound such as Sodium Silicate (Na2SiO3). Regardless of the type of bond, the metallic units will be diluted. Depending on the final briquette shape in conjunction with the raw material sizing, as much as eleven percent by weight of binder may be required to provide an adequate bond for handling without promoting deterioration or spalling. This results in eleven percent by weight of non-metallic units being blended into the briquette, and consequently the total percentage of metallic units is similarly reduced.
Briquetting assists with the previously set-out drawbacks of charging excessively small raw materials in the loose or as-produced condition into a cupola; namely, handling, metal yield, reduced melt rate, and recovery rates. The impact of briquetting on these noted concerns is as follows:
Handling
Both the hot pressed and bonded briquettes are extremely dense and packaged in a more manageable shape. This neutralizes the “handling” concerns such as storage space, movement frequency, pre-packaged costs and losses while in transport and held in inventory.
Metal Yield
No fines/chips are extracted/exhausted during charging into the pollution control system. Once again this is due to the density and shape of the charged product.
Reduced Melt Rate
As the briquette descends down the preheat segment of the cupola stack its shape is maintained. This condition eliminates any fines/chips from bridging (i.e., filling the voids and restricting upward gas flow) consequently leading to high back pressures and eventual blast reduction which promotes a loss of metal output.
The cold pressed briquettes perform similarly to the hot pressed briquettes except not to the same level of excellence since as noted earlier they are not quite as strong. Therefore, one can expect some continued handling losses and spalled-off segments of metallics being re-directed to the pollution system.
Recovery Rates
As a hot pressed or bonded briquette descends down the preheat zone of the cupola, it will eventually breakdown due to higher internal temperatures plus the stack burden above. For example consider the binder (i.e., cement or sodium silicate) portion that holds the bonded briquette together. The melting point of the binder is slightly less than 2000°F. which implies that it will begin to breakdown just above the “melt zone” in the cupola.
As the binder deteriorates the briquette reverts back towards its original shape (½″×down) and density. In this form the metallic units of the scrap are more prone to oxidizing due to an increase in surface area and direct contact with the upward flowing oxidizing gases.
In summation, the prior art method of briquetting enhances the recovery rate when compared to charging loose/fine raw materials; however, it does not approach the same levels as a single large piece of scrap such as a cast iron rotor or plate and structural.
There are several other miscellaneous yet important drawbacks associated with the briquetting of small sized raw materials in order to make it suitable for charging.
Rust Formation
Another important drawback applicable to both briquetted scrap and the more common scrap grades is the potential formation of “red rust”. This is difficult to eliminate as in most cases the material is stored outside at the suppliers end and is also exposed to the elements during both transport and while stored externally in inventory. The formation of rust simply means a loss of metallic yield or recovery even before one starts to process the material.
Flexibility
There are physical constraints to the number of different raw materials one can blend into a single briquette. For example, if there is a desire to briquette cast iron borings plus steel shot fines plus possibly a few alloy products such as ferrosilicon fines, and silicon carbide grain, then multiple briquette grades must be manufactured. It may be possible to blend a small percentage by weight of some non-metallic and/or metallic alloy to the main blend component of the briquette but it will be very limited. Differences in physical properties plus a limited space to charge an excessive number of different raw material components into the same briquette leads to an unstable product in terms of strength and chemical consistency.
Charge Weight Variability
Another important drawback applicable to both briquetted scrap and the more common scrap grades is the charge weight variability when charging the cupola. Most steel scrap is magnet-batched and it is not uncommon to observe charge weight variability in the order of plus/minus 50 to 500 pounds relative to a pre-set charge weight. Higher metal input variability obviously leads to higher metal output variability.